Objective  Narrow linewidth solid-state lasers are characterized by their excellent coherence and beam quality. Narrow linewidth lasers of certain wavelengths are necessary to meet the absorption or transmission requirements of specific ions, molecules, and materials. Therefore, it is of great significant to investigate the longitudinal mode characteristics of lasers at different wavelengths and operating modes. The 3-5 μm spectral range falls within the atmospheric window. Mid-infrared lasers in this band have been widely used for environmental gas monitoring, spectral analysis and optoelectronic countermeasures. Currently, MgO:PPLN crystals are typically employed in optical parametric oscillators (OPOs) to generate mid-infrared lasers within the 3-5 μm spectrum. This is attributed to their high second-order nonlinearity coefficient, large damage threshold, and widely tunable wavelength range. In addition to the tunability of the output wavelength, the optical parametric oscillation process also possesses the ability to suppress multi-longitudinal-mode operation within the cavity. While previous experiments have demonstrated that multi-longitudinal-mode operation can be suppressed by placing optical parametric crystals inside the cavity, However, more specific studies are limited. In this study, a comparative analysis of the variation of longitudinal mode properties at fundamental and idler frequencies was performed using MgO:PPLN crystals.

  Methods  The output wavelength at different temperatures is simulated based on the phase matching equation and the dispersion equation, as depicted in Fig.1. The experimental setup is illustrated in Fig.2. An fiber coupled 808 nm laser diode continuous-wave laser was used as the pump source, with a core diameter of 200 μm and numerical aperture of 0.22. A 1:2 focusing lens result in a spot radius of 400 μm at the Nd:YVO₄ crystal. The crystal has a dimension of 3 mm×3 mm×18 mm and a doping concentration of 0.3%. Plane mirrors M1 and M2 form a 1064 nm fundamental frequency optical resonator with a cavity length of 95 mm. A Q-switched pulse output of the fundamental frequency was obtained using an acousto-optic modulator. The fundamental frequency wave was directly coupled into the MgO:PPLN crystal via a 50 mm focusing lens, resulting in a beam radius of 400 μm for the fundamental frequency wave. The MgO:PPLN crystal, with dimensions of 10.5 mm ×1 mm×20 mm, a doping concentration of 5%, and a poling period of 31.0 μm was used. The OPO consist of plane mirrors M3 and M4 with a cavity length of 56 mm. The coating parameters of the lenses used in the experiments are presented in Tab.1.

  Results and Discussions   Figure 3 depicts the output power variation of the 1064 nm fundamental frequency wave with the pump wave. A Q-switched laser output with a maximum power of 7.03 W is obtained at a repetition frequency of 120 kHz. Figure 4 illustrates the variation of output idle frequency optical power with the fundamental frequency. At a room temperature of 20 ℃ and a fundamental frequency optical power of 7.03 W, an idle frequency light with an output power of 0.702 W and a wavelength of 3.196 μm is obtained, corresponding to a conversion efficiency of 9.95%. Figure 5(a) shows the time-domain waveforms of the measured fundamental and idle frequencies. The pulse width of the 3 μm idle frequency laser is 4.7 ns, which is slightly narrower compared to the fundamental frequency laser and has a smoother waveform. Fourier transforms are performed on the waveforms, as shown in Fig.5(b). It can be seen that the multiple longitudinal modes are significantly suppressed after the OPO process, consistent with the results observed in the time domain.

  Conclusions  By pumping the Nd:YVO₄ crystal with an 808 nm laser diode, multiple longitudinal mode output of the fundamental frequency wave with a repetition rate of 120 kHz and a pulse width of 8.1 ns was achieved, resulting in a maximum output power of 7.03 W. Based on this fundamental frequency pump source, an MgO:PPLN-OPO was developed, yielding a pulse width of 4.7 ns and an output power of 0.7 W for the 3 μm idler wave, with a fundamental-to-idler wave conversion efficiency of 9.95%. Comparing the Fourier-transformed temporal waveforms of the fundamental frequency and idler waves, we can clearly observe the suppression of higher-order longitudinal modes of the idler wave during the OPO process. This study has significant reference value for regulating the longitudinal mode characteristics in OPO and achieving low noise parametric optical output.